This invention relates to a method and apparatus for making reflectivity measurements of low-loss highly reflecting coatings, and transmission measurements of low-loss antireflection coatings on optical elements based upon the intensity decay time of an optical cavity which includes the elements. The method has utility in testing optical cavities such as those employed in ring laser gyroscopes.
Low-loss optical coatings often play a crucial role in the performance of optical instruments. Unfortunately, coating characteristics can vary considerably batch to batch from a single manufacturer and drastically manufacturer to manufacturer, even when the coatings are required to meet identical specifications. For this reason, it may be useful for a manufacturer or to a customer to have on hand apparatus which can measure coating characteristics rather than rely upon the specfications for the coating.
An object of this invention is to provide a method and apparatus for measuring the reflectivity of highly reflecting coatings (having reflectivity approaching 1), or to measure the transmission of low-loss antireflection coatings (on low-loss substrates). Seemingly straightforward techniques, such as measuring the small change in incident light intensity with a power meter, have proven unreliable. A technique for measuring reflectance suggested by Virgil Sanders, Appl. Opt. 16 19, (1977) works well for moderate loss optical elements, but less so when losses are very low. That technique measures total reflection loss of polarized light using two intralaser cavity measurements, one with the sample mirror and one without, by varying the angle .theta. of a rotatable window in the path of the beam in the cavity from Brewster's angle until the laser action is quenched in both directions. The difference in the included angle for the two measurements is a measure of the total loss in the sample mirror.
The method of the present invention, based on optical cavity decay time, is reliable, and in fact becomes more accurate with decreasing losses. Because of its accuracy, it is also useful in testing optical cavities, e.g., those used to implement what has come to be known as ring laser gyroscopes. The fundamental quantity measured is the cavity decay time. From this quantity, other characteristics, such as mirror reflectivity, can be inferred.
A different technique developed by J. M. Herbelin, et al., Appl. Opt. 19 144 (1980) and Appl. Opt. 20 3341 (1981), obtains the cavity decay time through a measurement of the phase retardation of an intensity modulated CW laser induced by an optical cavity. That technique transmits the intensity modulatd CW laser beam through the cavity, and measures any shift in the phase of the transmitted modulation as a measurement of effective photon lifetime in the optical cavity.